18 October 2011

Harnessing the power of fusing hydrogen isotopes together has long been a staple of science fiction, a means of achieving otherwise unachievable power densities, so as to make so many gee-whiz devices within the realm of the possible. In reality, fusion design concepts are generally massive, cantankerous, and incredibly expensive (see magnetic confinement fusion and internal confinement fusion). Fusion has been practical for awhile now, but it generally requires more energy input than it outputs, and the prototypes have been ludicrously expensive. The joke is with fusion, with research that's been ongoing for fifty years, is that it's always and always will be, "twenty years away."

In order to make hydrogen fuse, the first step is to get it hot enough that it ionizes, such that the electrons are no longer attached to a nucleus, forming the forth-form of matter, a plasma. Generally, the aim is to bring together a Deuterium (proton+neutron) ion and a Tritium (proton+2 neutrons) ion close enough together that the strong nuclear force affects the fusion of the two ions. However, the electrostatic charge on the ions is a longer range force, and it tends to mess up collision trajectories, such that only very high energy ions on a direct collision course could ever fuse. So the temperatures required are quite massive.

The magnetic confinement tokamak design that most people will be familiar with due to its widespread coverage in popular science magazines, tries to achieve more or less steady-state fusion power. Steady-state fusion tends to be plagued by energy losses, particularly turbulence in the plasma, that bleeds off power. In comparison, pulsed concepts like internal confinement are easier to initiate, but the natural tendency of an extremely hot gas is to expand rapidly, so fusion rapidly slows and stops, limiting the overall efficiency of the process.

Ok, enough background: enter General Fusion, a company based in British Columbia, that is angling to build a fusion power generator that, well, seems like it would fit right into a Steampunk science fiction novel! It's the one fusion concept that I've seen that one could conceivable build using relatively low-technology components: pistons, microwave ovens, that sort of thing. It's something MacGyver might build.

Magnetized Target Fusion (MTF) is a hybrid concept that is supposed to be low-cost. It was first proposed in 1976 as the LINUS concept, and it relies on first forming a small ball of deuterium and tritium plasma, called a plasmoid (or sometimes a spheromak). The plasmoid is given some angular momentum, such that it's actually a vortex, so that it has an inherent magnetic field that holds the plasmoid together for a brief moment. The plasmoid, which is already pretty warm, is then compressed so that a pulse of fusion occurs.

The main advantage of forming a plasmoid first, over the plain inertial design, is efficiency in transferring energy from electricity into the plasma. Lasers, plain and simple, aren't efficient at converting electricity to coherent light — I don't know what the lasing efficiency is at the National Ignition Facility, but commercial solid-state lasers are usually in the single digits. In comparison, a plasmoid can be formed with basically a high-tech microwave, using radio-frequency radiation, and the conversion efficiency is very very high.

Of course, the next question is, how to compress the plasmoid? A plasmoid has a lifetime of approximately 100 μs according to General Fusion, so compression has to occur on that timescale. The proposed solution is to use over two-hundred pistons driven by compressed air to smash into the 'pot' holding the plasmoid, inducing a converging acoustical wave. As the wave converges, its strength increases and it collapses the plasmoid to very high pressures, ~1 Megabar and results in a enormously high magnetic field within the collapsed plasmoid, on the order of 1000 Telsa. Effectively, it's like an artificial implosion nuclear bomb, using a very small amount of material. By using pneumatically-driven pistons instead of say, lasers, to achieve compression General Fusion is again gaining major efficiencies in terms of their energy input to output ratio (aka 'gain' in the fusion world). Air can be compressed relatively efficiently up to thermodynamic limits, so the whole concept doesn't have massively lossy steps that crush the overall system efficiency.

Since the pistons are basically flat, the shock wave will actually not be perfectly spherical. Also, it's pratically impossible to get all the pistons to hit the sphere at the exact same time — General Fusion claims they have accurate control of the impact time down to 5 μs which is 'good enough.' Since there will always be some error in the impact timing, the shock wave will imperfectly compress the plasmoid and one can expect a lot of cavitation and other hypervelocity fluid dynamical effects. The cavitation is similar to shaped-charge explosives, in that very high-speed jets are formed. I am not very clear on the physics of these plasma jets, but I would guess that they are basically the source of the ultra-high temperatures that make fusion possible with this concept. So cavitation early in the compression of the plasmoid is bad, because it bleeds off energy and reduces the ultimate compression achieved. However, a certain amount is probably desirable once the pressure reaches its ultimate limit.

Figure 1: General Fusion's pneumatic fusion reactor concept (http://www.generalfusion.com/generator_design.html). Plasmoids are formed in the plasma injectors (cones on the top and bottom) and then injected into the 'pot' of liquid lead and lithium. The two plasmoids collide in the middle and are metastable for a brief instant. The pot is surrounded by 220 pneumatically driven pistons which hammer the side of the pot, creating an imploding acoustical wave that compresses the plasmoid, causing a pulse of nuclear fusion.

The pot itself is actually full of a mixture of liquid lead and lithium metals, as there's a need for an 'aether' to transmit the acoustical energy. The lead acts as a neutron/thermal heat sink, absorbing the energy from neutrons produced by the fusion event both to recover it in the form of heat but also to protect the rest of the machine from high-energy neutron radiation. The lead-lithium mixture carries the heat produced by the fusion pulse to some working fluid (i.e. water) which can then produce electrical power.

The lithium is a slow neutron absorber, but it also undergoes fission to hydrogen and helium isotopes (via n + 6Li → T + 4He and n + 7Li → T + 4He + n), thus acting as a source of tritium, which is very expensive, radioactive, has a tendancy to leak through solid materials, and dangerous, since it can be used to make hydrogen bombs. Hence the reactor is designed to have a high breeding ratio (claimed at 1.6:1), so that once a little tritanium is given as a starter, more comes out.

Flowing the lead-liquid mixture in and out of the pot is likely a little tricky because the mixture has to spin in the pot, so as to setup favourable conditions for the plasmoid collision.

For the test-bed unit, which is smaller than an industrial scale reactor would likely be due to efficiencies of scale, about 100 Megajoules of mechanical energy is required as an input and about 600 MJ of thermal energy is produced. The heat can then be used to make steam, just like any other thermal power plant, and recovered at around a 33 % efficiency, so that 200 MJ of electrical energy is produced per shot. Hence the net would be 100 MJ per shot, and the target goal is 1 shot per second, thus producing 100 MW of power.

There are of course a variety of problems with the concept. One of the biggest is getting the two plasmoids to collide and combine in the desired manner to form a little vortex of plasma in the centre of the pot. This is a hard thing to test without two working plasma injectors and a pot of liquid lead-lithium. Currently they are relying on simulations, and there is plans for an explosive-based compression test to see if their plasma injector is working as desired. The disadvantage of the explosive-based method is that it's destructive, so they can only get one test per boom-boom. This makes iterating the design expensive and manpower intensive, but they are planning a shot in the fall of 2012 without Tritium.

Another problem is that material from the pot or the plasma injector nozzles (called spalling in the tokamak field) will be absorbed into the lead-lithium liquid, and that these impurities will radically increase the rate at which the plasmoids dissapate.

Irradiation of the machine itself is also a problem. The lead-lithium matrix will absorb 99.9999 % of the neutrons but the walls of the vessel will still become too radioactive after about six months of use. Fortunately neutron embrittlement should not be a problem because the neutrons should be moving at relatively low velocities by the time they get to the shell of the pot.

Lifetime of the shell and pistons is also a concern, due to the thermal and shock stress caused by the impacts. This is actually something that improves as the machine gets bigger, because the pistons can move slower in order to achieve the same overall compression ratio.

When I describe this concept as being steampunk-themed, I am exaggerating a bit. In fact, this concept requires exquisite timing to control all the pneumatically driven pistons, and to form and inject the plasmoids into the liquid lithium-lead chamber, and that means lots of fibre-optics and other high-speed network devices unavailable 20-30 years ago. It is definitely the hipster of fusion power schemes, however.

The bottom line: I remain skeptical that nuclear fusion can be more economical than either photovoltaics, which will eventually be the cheapest source of power on the planet, or advanced fission reactors. Fusion is one of those gee-whiz things that sounds really exciting, until you start getting into the details and wonder how it will be economical, and the radiation waste aspect isn't really any better than fission (there's no worry about products decaying into Radon, which is a radioactive gas, but they do have to worry about Tritium contamination of the reactor, and it's a gas that can flow in-between the molecules of solid metal). The company has raised about $40 million thus far, and they probably need more than double that to finish their prototype in 2013/14, so it will be interesting to see if they find it. On the other hand, this concept is ripe for science-fiction fodder.

03 April 2011

So the other day I decided to make myself brunch, for which I wanted to make something to go with poached eggs. Now, poached eggs boiled in vinegar water four minutes each are a a little runny with the yokes only semi-congealed on the edges, so they need something starchy to sop up the yoke. Hashbrowns could work, but I had some purple potatoes that tend to bleed their pigment, so instead I thought I'd bake them. Then my next brainwave was to wrap them in bacon and chiles, and damn they were tasty.

Ingredients

Two small purple potatoes, half lengthwise

Four strips bacon

(optonal) Pickled green Thai chiles

Salt and pepper, to taste

Olive oil

Slice the potatoes in half, then salt and pepper them. The potatoes were small, not fingerlings but not nearly full-sized baking potatoes. Wrap the bacon around loosely so that the top is well covered. Insert the chiles, and drizzle olive oil on top as 'starter fat' to prevent the bacon from burning. Bake at 350 °F for about 45 minutes, until the potato is cooked through. The bottom of the potatoes should come out nice and golden, and the bacon will shrink wrap around the potatoes. Next time I might microwave the potatoes a bit first to decrease the cooking time and to cook the bacon a little less.

28 January 2011

So least you readers think I am just being lazy in not posting, here is some photos from an avalanche safety course I took last weekend. We were at Bow Summit for the practical (skiing) portion of the training, here's some pictures sans people.

Sunset the night before.

The skin track up.

The snowpack is about 70 cm of wind-loaded slab on top of 80 cm of very weak sugary surface hoar. The interface is obvious in the picture. In other words very dangerous for slab avalanches. The snowpack was surprisingly strong given how bad it looked (still very dicey over 30°), probably because the top slab was still fairly plastic. We didn't hear any whumpfting (audible evidence of snow settling when skied over).

The view across the valley as the clouds broke up. Ski touring is totally unlike lift-served skiing in tone.

Physical activity should be fun play-time. I don't really get the emphasis on weight lifting in the paleo community, as it seems very boring to me.

04 January 2011

Ah, the immune system. It is what makes trillion-cell organisms possible. Immune system cells actively patrol the body and attack bacteria, fugii, and parasites. In the process, they often causing collateral damage to self-tissue. The innate immune system compared to the adapted immune system is evolutionarily older and more prone to carpet bombing tactics to defeat pests. As such, when it doesn't operate properly, a broad list of symptoms can present. The innate immune system actually has two roles: both fighting off foreign cells but also repairing damaged tissue. When the innate immune system is always fighting and never repairing it leads to the state of chronic inflammation at the heart of many of the diseases that plague modern civilization, like diabetes, stroke, and heart disease.

What then determines which role an innate immune system cell body operates in at any given time and place in the body? Let's look at monocytes/macrophages.

Monocytes are undifferentiated (i.e. unspecialized) immune system cells of the innate immune system that circulate in the bloodstream. In response to chemical signals from the tissues adjacent to their blood vessels, they enter into the tissue to either fight infection or repair tissue damage. When monocytes enter tissue, their gene expression causes them to become more specialized and they are then called macrophages (they can also become other immune system cells). There are two basic phenotypes for macrophages, which are essentially the ying and yang of the macrophage community.

M1 macrophages are pro-inflammatory and fight infection. They are the classical state for macrophages that you would find described in a textbook. Primarily, they detect and fight foreign organisms (viruses, bacteria, and parasites). They are characterized by the production of pro-inflammatory cytokines, chemicals which alert the other cell types of your immune system to react and destroy the invader (as well as adjacent 'self' cells).

M2 macrophages are anti-inflammatory and repair tissue damage. For example, when you exercise and your muscle tissue is damaged, it is M2 macrophages that infiltrate your muscle organs and affect the repairs [Tidball, 2010] after the initial M1 surge. The characteristic cytokine of M2 macrophages is interleukin-10 (IL-10), which encourages other macrophages to enter the tissue and differentiate into M2 phenotype but also discourage the attention of cyto-toxic 'killer' cells from the lymphocyte family of the immune system.

The differentiation of macrophages, from M1 to M2, is not all that distinct and is generally though to represent the two extremes of a continuum. My reading suggests macrophage populations can make the transition from one phenotype to the other, without die-offs. This is probably a bad thing for chronic modern diseases, in that many of the diseases that are as a result of macrophage dysfunction occur when apoptosis (programmed cell death) is impeded.

One of the beneficial effects of eating a diet low in inflammatory factors (e.g. fructose, wheat, smoking) is that the overall levels of pro-inflammatory hormones, such as cortisol or interferon, are low so the transition from high M1 expression to high M2 expression can occur more rapidly. I strongly suspect this is why most people who transition to the paleo-diet are much better able to put on muscle mass. As the Tidball article indicates, chronic exercise is another no-no because it doesn't give enough time for the M2 macrophages to enter and affect repairs, so the muscle is always in an inflamed state.

This doesn't, in general, appear to actually reduce the ability of the body to fight infectious disease, however. This is probably because even though the overall inflammatory condition (as determined by circulating cortisol globally or cytokines locally) is low, it can easily spike when a foreign body attacks. On the other hand, actual conditions where adrenal functions are suppressed (i.e. Addison's disease), the immune system is hamstrung by the homonal milieu it finds itself in and the immune system doesn't function properly to defend against infection. One hypothetical cause may be that the ratio of M2 to M1 is tilted in favour of M2 in adrenal insufficiency.

One pathogen that is known to mess with the M1/M2 expression in macrophages is human cytomegalovirus (HCMV). HCMV basically takes M2 macrophages or undifferentiated monocytes and reprograms them to be more like M1 macrophages in some ways (Chan et al., 2009 and Chan et al., 2008). Importantly, they do not become like M1 macrophages in that they continue to release interleukin-10, which discourages the adapted immune system from deploying 'killer' lymphocytes (natural-killer cells and T-cells) but encourages more M2 macrophages (red shirts, basically) and at the same time they up-regulate the production of protein filaments that make macrophages motile, so that they can better travel about and infect other organs. So let's all remember, whenever you have massive infiltrate of an organ by macrophages, you can probably bet there's too much IL-10 being produced.

To me the various pathogens that excel at molecular mimicry to hide from the immune system seem to be playing quite the bogey-man role in a whole host of chronic diseases. These are principally the Herpes virus family (which also includes cytomegalovirus and the Epstein-Barr virus which has been linked to chronic fatigue syndrome) and the bacteria C. Pneumoniae, which has been linked to atherosclerosis and a whole host of other chronic diseases. Paul Jaminet over at Perfect Heath Diet has also been covering C. Pneumoniae in the brain,

It also seems a lot of disease symptoms occur when macrophages manifest some combination of the M1 and M2 state. Consider that glucose in the blood is regulated by the liver predominately, so insulin resistance (aka metabolic syndrome) reflects failure of the liver to be able to do it's job. Largely, this is blamed on 'inflammation' in the liver, e.g. alcholic and non-alcoholic fatty liver disease, but what does that mean exactly?

A review (Olefsky and Glass, 2010) states that macrophages in tissue that produce excessive quantities of tumour necrosis factor-α (TNF-α) and interleukin-16 (IL-16) (both pro-inflammatory cytokines), to the point that they are detectable in the blood. I like the following quotation from the review, as it really lays out the problem:

The discovery that adipose tissue from obese mice and humans is infiltrated with increased numbers of macrophages provided a major mechanistic advance into understanding how obesity propagates inflammation (4, 5). Adipose tissue contains bone marrow–derived macrophages, and the content of these macrophages tracks with the degree of obesity (4, 5, 31, 32). In some reports, greater than 40% of the total adipose tissue cell content from obese rodents and humans can be composed of macrophages, compared with ~10% in lean counterparts (32).

That nearly half of fat tissue mass is actually not fat cells, but immune system cells is kind of amazing to me. A very similar thing happens in liver disease. Now, reference #32 is a mouse study (Weisberg et al., 2006) but it in turn cites two other mice studies that are more pertinent (Weisberg et al., 2003 and Xu et al., 2003). Both articles show that gene expression for various proteins that attract immune system cells are strongly up-regulated in the adipose tissue of fat mice. The question is why? Is it diet? I suspect partially, but the revelations regarding what cytomegalovirus can do to macrophages makes me suspect latent pathogens are attracting macrophages as lambs to the slaughter. Are lab mice susceptible to chronic infections given their conditions and short lifespan? Are these latent viruses transmitted from mother to infant?

The prevalence of immune system bodies in the adipose tissue of the obese mice illustrates an example of the, "diseases of civilization," being largely driven by dysfunction of the innate immune system, probably egged on by latent viral and bacterial infections and an unnatural diet. The pieces of the puzzle are mostly there now and evidence will continue to accumulate until we have a better view of the whole picture. Stop the sources of inflammation (i.e. immune system activation), give the immune system the substrates it needs to fight effectively, and the other symptoms will go away.